This invention relates to stable oscillators and in particular to a low noise L band dielectric resonator stabilized microstrip oscillator having ultra low phase noise, low 1/f noise corner and constant power and frequency over temperature.
Lower phase noise in oscillators with constant frequency and power over temperature is required in new emerging systems for improved dynamic range, simplicity, lower cost, and improved spectral purity. A fixed tuned stable oscillator with these desired features with low 1/f noise is a key element in modern stalo's. Such a source can be used in conjunction with a crystal multiplier source in an "active phase locked filter" to reduce crystal multiplier noise by as much as 20 dB. It can also be used as a primary source due to the accurate frequency characteristics.
In the past, oscillators usually consisted of a large coaxial cavity with coupling to the transistor, varactor, output, and frequency tuners. The theoretical Q of coaxial section is high, but with insertion of all the coupling elements, the actual Q is low (.about.500), and the circuit is generally very temperature sensitive requiring 2 varactors, one for temperature correction, and one for phase locking applications. This type of oscillator tends to be very expensive requiring critical assembly. An example of such a device is the E3A oscillator which in its day was state of the art but does not suffice for lower phase noise system requirements. The 1/f noise corner of this unit is, as well as most oscillators, approximately 100 KHz. Lower corners giving rise to lower phase noise at lower offset frequencies is highly desireable.
A dielectric resonator stabilized oscillator (DRO), on the other hand, has the capability of higher Q.
Such a device has been developed by Bell Laboratories and disclosed in detail in the publication of G. D. Alley and Han-Chiu Wang entitled An Ultra-Low Noise Microwave Synthesizer, IEEE Transactions Microwave Theory and Techniques, Vol. MTT-27, No. 12 December 1979, pp. 969-974. The device disclosed thereby has excellent noise characteristics but the transistors and resonator material are not available nor has this unit demonstrated stability both amplitude and frequency over large operating temperature ranges with large varactor modulation sensitivities.
A review of the current state of the art reveals that there are no known L-band DRO's that use commercially available transistors and resonator materials for low phase noise, low 1/f noise corner, constant frequency and power over wide operating temperature with a varactor tuning port.
It is apparent from the foregoing discussion that there currently exists the need for a stable oscillator having improved phase noise, phase corner and constant power and frequency over temperature characteristics. The present invention is directed toward providing an L-band DRO that will satisfy that need. In accomplishing that end it will be seen that the L-band DRO of the invention, hereinafter described in detail, is significantly different than any other reported DRO. Its noise is 20 dB better than the coaxial type units (E3A) with a 1/f noise corner one tenth of the E3A device, providing even lower noise at low offset frequencies.